This invention relates to a signal reception system comprising an adaptive filter and a differential detection circuit.
Generally, such a signal reception system comprises a demodulation circuit, an adaptive filter, a differential detection circuit, and an error signal generation circuit. The demodulation circuit demodulates a reception signal into a demodulated signal. The reception signal is carried by a carrier wave through a transmission path. The transmission path has transmission characteristic which varies with a lapse of time. The adaptive filter has a plurality of filter coefficients. The adaptive filter carries out compensation operation of the transmission characteristic by the use of a high speed adaptive algorithm and produces a filtered signal. The differential detection circuit carries out differential detection operation in the manner known in the art. In the differential detection operation, a delayed signal is generated by the use of the filtered signal. Namely, the delayed signal is derived from a delayed operation that the filtered signal is delayed by a single symbol duration. The delayed signal serves as a standard of the carrier wave. The differential detection operation is carried out by the use of the filtered signal and the delayed signal.
As a result of the differential detection operation, the differential detection circuit produces a synchronized signal synchronized with the phase of the carrier wave. The error signal generation circuit generates an error signal representative of an error between a predetermined reference signal and the synchronized signal. The adaptive filter is supplied with the error signal and adjusts the plurality of filter coefficients in accordance with the error signal so that a root mean square of the error becomes a minimum value. By the use of the high speed adaptive algorithm, the error can be converged in high speed.
In such a signal reception system, when the delayed signal has an extremely small signal value which closes to zero, phase information included in the delayed signal extremely decreases. This means that the synchronized signal has a degraded reliability. In this case, it often causes a phenomenon that the error of the error signal becomes large because the synchronized signal has the degraded reliability. As a result, the plurality of filter coefficients are updated by error. In the high speed adaptive algorithm to which the error is converged in high speed, convergence of the high speed is degraded on occurrence of the above-mentioned phenomenon because update of the plurality of filter coefficients depends heavily on the error of the error signal.